The present invention refers to a measuring device for measuring the elastic properties of a surface structure, as defined in the preamble of claim 1. Such a measuring device is known from EP 0 329 817. The known device comprises a probe supported for axial movement in a housing, the measuring tips of the probe being adapted to be orthogonally applied to the skin surface of a biological body, using a predefined contact pressure. The measuring tips transmit sound pulses from bi-morph piezo transducers into the skin surface, with one measuring tip acting as a transmitter and the other two acting as receivers. The receivers measure the time span between transmission and receipt of the acoustic pulse traveling through the skin between the measuring tips. From the time measured, the propagation velocity of the sound wave in the skin is calculated.
The flat and strip-shaped bi-morph piezo transducers consist of a ceramic material provided with electrodes and having low breaking strength. The bi-morph elements are thus very brittle and will break soon under mechanical stress.
It is the object of the present invention to provide a measuring device of the type mentioned above that has greater operational safety and is insensitive to impacts.
Advantageously, the invention provides that the measuring tips are formed by the holder for the bi-morph elements, the strip-shaped bi-morph elements being fixed to the holder only on one side and by one end. By making the holders of the bi-morph elements form the measuring tip themselves, the bi-morph elements are advantageously subjected to no mechanical stresses so that they will not break even when treated improperly or when subjected to high impact stresses.
Preferably, the bi-morph elements are fastened to the end of the holder facing the measuring tips. The holder allows for a transmission of acoustic pulses via the measuring tips onto the surface structure and simultaneously protects the bi-morph element against damage. The strip-shaped bi-morph elements are each fixedxe2x80x94preferably by glueingxe2x80x94to the holder at the free end thereof facing the surface structure, whereby an optimum transmission of the generated acoustic pulses onto the free end of the holder forming the measuring tip can be obtained.
At the respective free ends of each strip-shaped bi-morph element a seismic mass is fastened, preferably by glueing. When the bi-morph element of the transmitter deflects due to an electric pulse, the bending moment is transmitted as an acoustic pulse via the measuring tip onto the surface structure. This bending moment is supported by the seismic mass with its inertia. The inertia of the seismic mass causes a very sensitive reaction of the arrangement to fast pulses, yet hardly any reaction to the relatively slow movements caused by improper handling.
Preferably, the holders have a longitudinal recess adapted to the bi-morph elements in which the bi-morph elements can vibrate freely. The recesses allow the bi-morph elements extending parallel to the holders to vibrate through the recess when vibrating orthogonal to the holders.
The holders consist of two flat, strip-shapes members having their ends opposite the measuring tips fastened to a common support with a parallel distance therebetween. The holders are very flexible so that the acoustic pulses of the transmitter are not transmitted to the support. The preferred material for the holders is Makrolon. The flexibility of the holder inhibits transmission of the acoustic pulse onto the receiver via the suspension.
The probe is supported for axial movement in the housing and the measuring tips can be applied against the skin surface with a defined spring tension.
The acoustic pulse and the measurement can be triggered when the probe has traveled a predetermined path. This guarantees that the measuring tips are always pressed against the surface structure with the same contact pressure.
The acoustic pulse and the measurement may also be triggered after a predetermined time delay, which can also be combined with the signal""s having traveling a certain probe path.
Preferably, the transmitter consisting of a flat bi-morph element couples a single acoustic pulse into the surface structure.
The measuring signals picked up by the receiver can be evaluated by the measuring means using resonance measuring in the frequency range between 0 and 10 kHz.
In a development of the invention, it is provided that the probe has a guiding member fixable to the surface structure to be examined, in which guiding member the probe is guided orthogonal to the surface structure and in which the probe can assume different defined angular positions using marks at he probe and at the guiding member. The guiding member, which may be a cylindrical sleeve, for example, glued to the surface structure to be examined, receives the probe such that an orthogonal orientation of the probe relative to the surface structure is ensured. Moreover, the circumference of the guiding member is provided with angular marks that, together with a mark on the probe, allow to rotate the probe through predetermined angles so that measurements can be made under an angle of 0xc2x0, 45xc2x0, 90xc2x0, or 135xc2x0, for example, relative to an initial position.